The present invention relates to a filter which is improved in the performance of filtering fine particles. In one aspect, the invention relates to a filter made from nonwoven fabric.
Heretofore, woven fabric and nonwoven fabric have been used as filter media. Because of its unique structure, nonwoven fabric is more effective in removing fine particles in a gas and liquid than woven fabric. Being formed by bonding or entangling fibers mechanically, chemically, and/or thermally, the nonwoven fabric has randomly arranged gaps among fibers.
Nonwoven fabric filters are prepared by a variety of techniques which include the following:
(1) combining nonwoven fabrics of different thickness and density, PA1 (2) converting the needle punch surface of a nonwoven fabric of specific density into a filmy state, PA1 (3) forming a coarse part and a compact part in the same nonwoven fabric, or PA1 (4) imparting a density gradient in the thickness direction.
Filtration by the filters produced by these techniques is achieved by a physical mechanism which involves either deposition of comparatively large particles on the surface of nonwoven fabric or the accumulation of particles across the entire thickness. These filters, however, are not capable of filtering out extremely fine particles in the range of from 0.05 to 1 .mu.m; such fine particles pass through the clearance, and openings among fibers.